Abstract
In this paper, the general ordinary quasi-differential expression tau of nth order with complex coefficients and its formal adjoint tau(+) are considered, and a number of results concerning the location of the point spectra and regularity fields of products of the operators which are generated by such expressions are obtained. Some of these are extensions or generalizations of those proved for symmetric case in [1, 12, 13, 14, 15], whilst others for general case in [9, 10, 11].